This application claims the priority of German Patent Document 199 50 205.6, filed Oct. 19, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a control system for variable torque distribution for a four-wheel-drive vehicle.
A control system of this type is described e.g., in German Patent Document DE 40 02 821 A1 wherein a torque distribution device changes the torque ratio between the front and rear wheels by appropriate control of a friction clutch as a longitudinal lock. By determining a torque distribution ratio, both the driving performance and the traction of a vehicle can be considerably influenced. A similar control system is also known from European Patent EP 0 911 205 A2. However, controllable clutches as longitudinal locks are complicated, heavy and expensive.
Designing a wheel-selective brake intervention on one wheel where the slip has occurred in a four-wheel-drive vehicle is also known from DE 40 02 821 A1. As a result of this structure, stability control does not occur until an instability has already occurred. In this process, there is a problem that must be solved, in that the reference vehicle speed for recognition of a slipping drive wheel cannot be determined as usual on the basis of the circumferential speeds of the wheels that are not driven.
In addition, the use of high torque distribution ratios (Torque Bias Ratios TBR=Torque_high/Torque_low) means problems in the calculation of the reference speed for slip control systems and also high levels of tension in the drive train when driving around curves.
In an object of the present invention to provide an improved control system for changing the distribution ratio and having increased safety and a reduction in cost and construction space.
According to the present invention, a non-variable longitudinal lock with specified torque distribution ratio is provided with a control system for variable torque distribution for a four-wheel-drive vehicle with a brake control system through which a wheel-selective brake intervention is possible. To change the torque distribution ratio, a symmetrical brake intervention occurs on the wheels of the axle, whose torque transmission percentage is to be reduced in relationship to the other axle, if no slip is present.
Brake control systems for wheel-selective brake interventions are known, especially as a combination of anti-lock brakes, traction control and driving dynamics control systems. A symmetrical brake intervention means that the same braking torque is applied to both wheels of the axle whose torque transmission percentage will be reduced in relationship to the torque transmission percentage on the other axle.
Because the brake intervention can be adjusted without slip, the advantages of a variable torque transmission ratio can be achieved in such a way that the stability is increased even before the occurrence of an instability, according to the requirements in various drive conditions. For example, the torque transmission percentage at the wheels of the rear axle is increased during drive-away or the torque transmission percentage at the wheels of the front axle is increased when driving around curves. However, the adjustment of the torque transmission ratio can not be implemented exclusively by brake interventions because loud noises and high tension in the drive train can occur due to continuous regulation. These disadvantages are eliminated by the fundamental use of a simple longitudinal lock with the usually required fixed, base torque transmission ratio. The change in the torque transmission ratio because of the brake intervention will be carried out, if necessary, e.g., depending on the steering angle, the wheel load, the coefficient of friction at the individual wheels, the yaw angle, the yaw angle speed, the float angle, the lateral acceleration, the slope and/or the pitch and on other operating parameters that describe the momentary driving status.
In a first alternative according to an advantageous further development of the invention, the engine torque is automatically increased in an amount that corresponds to the total braking torque of the brake intervention, in order to avoid any reduction in the total traction torque.
In a second alternative according to an advantageous further development of the invention, the driver receives a recommendation, preferably visual, acoustic or haptic, for engine torque reduction until an engine torque increase is achieved by corresponding actuation of the accelerator pedal in an amount corresponding to the total braking torque of the brake intervention. If a driver should want to avoid an automatic engine torque increase, he can decide if he considers a driving torque increase necessary.